In October, There Will Be 6 Billion People Alive On Earth -- Twice As Many As There Were Just 39 Years Ago. How Large A Population Can The Planet Sustain?

As if Y2K weren't trouble enough, the world now has to think about Y6B.

Barring the unforeseen, the world's population will click past 6 billion on Oct. 12. Hence, this year has been christened Y6B -- the year of 6 billion -- a remarkable milestone in human history. It's also a potentially troubling one.

Some fear that growing population will strain the world's resources, exacerbating problems from crowded classrooms and traffic congestion to water shortages and species extinction. Others welcome the new additions to the human family.

``The world's never seen anything like this,'' said Robert Engelman, vice president for research at Population Action International, a group that encourages family planning to slow the population surge.

``This is an unusual moment in demographic history . . . but in some sense it's a victory,'' Engelman said. ``We've succeeded at conquering disease enough and feeding ourselves enough as a world that we've been able to keep 6 billion people alive on the planet. But we're increasingly seeing signs of stress, and it's particularly notable that so much of the change is happening fast and in this century.''

It took all of human history up to 1804 for the world's population to reach 1 billion, say population experts at the United Nations. But it took about one generation for the population to double from 3 billion in 1960 to 6 billion this year.

Experts say Earth is gaining 78 million people annually -- roughly the equivalent of adding a city of 645,000 every three days.

Can Earth support us all?

``It depends,'' Engelman said. ``The reality is we don't understand the Earth enough to have a clue.''

PAI and other groups such as Zero Population Growth, a grass- roots organization focused on population control, warn against complacency. An expanding population, they say, is affecting the planet in ways we cannot quantify.

``It's about more people trying to fit into a particular space, one piece of real estate called planet Earth that we all share,'' said Tim Cline, spokesman for ZPG. ``The kinds of things we see everyday that add stress to our lives are brought on by a growing population.''

ZPG and others blame population growth for such esoteric losses as decreasing quiet space where people can go to be alone. Some researchers warn that the population surge will bring on more life-threatening problems such as:

* Rapid species extinction. With an increased need for farmland for food and acres for housing, humans are stealing space from plants and animals, disrupting ecosystems and threatening biodiversity. As some species are killed off, we will lose animals needed to pollinate crops, create oxygen and perhaps give us cures to diseases.

* Water shortages. Today, less than three-tenths of the world's water is available for human use. Fresh water is scarce for a half-billion people in developing countries such as India, the world's second- most-populous country behind China.

PAI projects that by 2050 -- when world population is expected to reach almost 10 billion -- at least one in four people in 48 countries likely will face acute water shortages. And water shortages threaten to reduce the food supply by more than 10 percent, says the Worldwatch Institute, a public policy think tank that focuses on global problems. Widespread hunger, civil unrest, and even wars over water could result, the group warns.

* Global warming. Scientists disagree on the extent of the human contribution, but they know that the Earth's hottest 15 years on record have occurred since 1980. Some say it's the result of humans releasing more greenhouse gases, especially carbon dioxide, through activities from burning trees to driving cars. Scientists who believe that human activity is a significant factor in global warming say that as the Earth's climate changes, we can expect more destructive storms and flooding, as well as melting ice caps and rising ocean levels.

Worldwatch said that last year was an ``off-the-chart'' year for record-high temperatures, which powered more destructive storms -- including two of the most powerful out of the Atlantic Ocean, hurricanes Georges and Mitch. Record storms and floods last year drove more than 300 million people from their homes, the group's researchers said, and caused $92 billion in weather-related damage worldwide, up 53 percent from 1996. Some suggest that the events of 1998 were not an aberration, but a window on the future.

``We're not saying that population growth is the sole cause of all these problems, but it's clearly one of the problems,'' Engelman said.

Other groups such as STOPP International (for Stop Planned Parenthood) scoff at the warnings and have their own campaign against population control.